Friends tell you that on a recent trip their average velocity was +20 m/s. Is it possible that at any time during the trip their velocity was -20 m/s? Explain

Yes , it is possible .

Explanation:

Suppose they travelled from A to B , a distance of 800 m at a velocity of

40m /s and then come back on the same path covering a distance of 200 m at the rate of 20/ s .In other words their velocity in their return journey was -20m /s .

Net displacement = 600 m

Total time taken = (800 / 40) + (200 / 20)

= 20 + 10 = 30 s

Average velocity = net displacement / total time

= 600 / 30 = 20 m /s .

Hence average velocity of 20 m /s is possible .  

it is the second option.